1. Field of the Invention
The present invention belongs to a technical field of a liquid crystal panel, an optical pickup, and an information reproducing apparatus.
2. Description of the Related Art
Conventionally, as a liquid crystal panel for compensating wavefront aberration, a liquid crystal panel having a construction in which electrodes are disposed on both sides of a liquid crystal layer is generally used.
This liquid crystal panel is arranged so as to compensate wavefront aberration, which is caused by inclination of the optical axis, by varying the refractive index of the light beam transmitted through the liquid crystal, and utilize the change of orientation of liquid crystal molecules constituting the liquid crystal depending on the voltage applied to the liquid crystal.
Specifically, the conventional liquid crystal panel has a constitution in which the inclination of the optical axis is cancelled out by varying the applied voltages with respect to each part of the liquid crystal to vary the refractive index relative to the light beam, thereby providing phase difference different with respect to each part of the liquid crystal to the light beam, and varying the optical path length of the light beam to the information recording surface. For example, the conventional liquid crystal panel has transparent electrodes constituted by a combination of plural partial transparent electrodes for applying a voltage to the liquid crystal, and applied voltages are varied with respect to each of the partial transparent electrodes, and thereby, phase differences different with respect to each part of the liquid crystal are provided.
In the conventional liquid crystal panel, forms of plural partial transparent electrodes within one transparent electrode are generally determined on the assumption that the position of the central axis of the objective lens and the position of the central axis of the liquid crystal panel are aligned with each other, that is, on the assumption that the center of the pupil plane of the objective lens and the center of the transparent electrode are aligned with each other.
On the other hand, in the general construction of an optical pickup, the objective lens and the liquid crystal panel are often constructed separately, and, in the optical pickup, sometimes the position of the central axis of the objective lens and the position of the central axis of the liquid crystal panel have been already out of alignment due to problems of its manufacturing process.
In addition, when actually recording or reproducing information to a recording medium, the so-called tracking serve control is sometimes performed to the irradiated position of the light beam on the recording medium, for example. In this case, the objective lens is moved in the direction perpendicular to the information track formed on the recording medium by an actuator etc. Accordingly, even in the case where misalignment has not occurred in the manufacturing process, sometimes misalignment inevitably occurs between the position of the central axis of the objective lens and the position of the central axis of the liquid crystal panel.
Therefore, as measures against the misalignment, the following two technologies are mainly proposed.
The first technology is a technology of preparing two patterns of electrodes of the liquid crystal panel in advance, and thereby, the tolerance to the misalignment with the objective lens is improved (for example, see publication of Japanese Patent Application No. 2001-143309).
The second technology is a technology of dividing transparent electrodes in advance so that the wavefront aberration may become least when predetermined misalignment occurs (for example, see Publication of Japanese Patent Application No. 2000-90479).
However, when the above described misalignment occurs, in the conventional liquid crystal panel formed on the assumption that the misalignment does not exist, the wavefront aberration can not be compensated sufficiently.
In addition, in the technology as the first measure against misalignment, since two patterns of electrodes are required to be prepared as a measure against misalignment in advance, patterning and control become complicated, and costs largely increase.
Further, in the technology as the second measure against misalignment, since electrodes are set so that the wavefront aberration may become least when predetermined misalignment occurs, the aberration compensation function when no misalignment occurs becomes degraded.